As the title states I'm having problems building up any pressure with my brakes. When I got the car a month and a half ago I didn't notice this problem at all, last time I ran it I noticed it didn't feel right. But now it is awful, it almost can't hold the car back at idle. Before I try to fix any other problem I'd like to have this working correctly. At first I thought it was just something on my front rotors, but I just packed the bearings and went out of my way to make sure the rotors were oil free when I put it back together.

Here is what I know so far. Car only stops after pedal goes down a few inches, from that point it cannot be moved further. However, it will stop the car like that. When I MASH the brakes the rear tires sound like they lock up, but the fronts don't seem to be putting any effort into stopping. There is approx 19" of vacuum at idle. Like I said in the title, even with the car off the brake pedal doesn't harden up. If I pump the pedal enough I will sometimes get a little bit of pressure, but it just fades away in an instant. The rotors are now clean metal on all 4. I've never changed the brake fluid in it, knowing the last guy he probably didn't either. So I assume it has been about 3-4years since it has been changed. Is it possible the fluid has froze? Or is it likely an internal seal has quit working and it needs to be replaced?

Thanks,
Jeff M.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Yep, look's like a kit job to me....
For more advice go here

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http://w3.uqo.ca/gravelle/images/250S/flood.jpg
DAN
1967 250s The Monster Project a.k.a "The Monster" a.k.a "Rolling Coffin" --sold--
The photographic ART thread
+++Price Guide+++

Could be the booster.

I would bleed the entire system before I do anything. If the problem has gotten worse over time, it may not be air in the system, but it's the cheapest place to start. If the fluids have not been changed in 3 years, there is likely water in the system, and fluid which is less than optimal. Check for leaky calipers when you do (they seem rare on a MB).

If the fluid is old, seals are probably gone. From what you described, I'd swap the master cylinder (you could rebuild it, but if you have never done one, get help). A new mc will give you peace of mind. If the brakes don't feel nice and firm and pedal travel is still variable after that, probably time for new or rebuilt calipers as well.

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Peter
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K

Just a couple weeks ago the brakes on this car was better than my 84's brakes. It had a really soft pedal, but the brakes were still fairly grippy. Then they dwindled away to this....

It has been getting down to single digit temperatures (F) here almost every night now. No leaky calipers, or lines. Actually speaking of leaks, I see the paint on the booster assembly has peeled off due to a previous leak apparently. Looks like it must of gushed out from where the MC meets the booster, it is all dry now though. The fluid itself looks clean, unlike my when I bought my 84, swamp water.... The MC itself looks mighty simple, however I didn't see the rebuild kit in fastlane. Where do I find it?

Doesn't the brake booster just make it easier for the driver? With a bad booster I would think the pedal would be very hard no matter if the car was running or not.

~jm

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Exactly, hard pedal = booster malfonction or vaccum leek.
I had that problem and master falure shortly after

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http://w3.uqo.ca/gravelle/images/250S/flood.jpg
DAN
1967 250s The Monster Project a.k.a "The Monster" a.k.a "Rolling Coffin" --sold--
The photographic ART thread
+++Price Guide+++

Bet it's the master and not the booster.
Symptoms match perfectly.

Best of luck.

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1986 300E 5-Speed 240k mi.

The reason it is probably hydraulic related rather than booster related is that you said the pedal goes down a few inches and no further, but it does not stop the car. If there is air or no fluid, this is what will happen regardless of boost. It sounds like you are feeling the pedal mechanically bottomed out.

It only costs a few dollars for a quart of brake fluid. Flush THOROUGHLY with fresh brake fluid and then assess the system. The flushing will be a good thing to do even if you end up rebuilding the hydraulics.

You are most likely correct in paying your attention to the m/c first. Look at all the wheels and cylinders and see if there is fluid anywhere. If not, then the m/c is almost certainly the problem.

Henceforth, flushing the entire brake system with a whole quart of fresh fluid once a year will virtually eliminate these problems from occurring.

Good luck,

Thanks to everyone for their help on this. While it makes no sense to me how it happened, but somehow air got in the passageways. I bled out one front wheel and instantly got a pedal back. So air must of been forming in the master cylinder?! I just hope it doesn't come back. I thought you had to run it dry before it could do that??!

I haven't bled all of the individual calipers out yet, but it has fresh fluid with a great pedal now. Back to my previous schedule of putting new boots on the CVs....

In the spring I may add. The P/O of my 83 didn't lubricate anything but the motor IMO, so I'm changing it all before I take it on the roads. The linkage for the accelerator would stick 2/3 open because the gross lack of grease....

On a whole different subject would anyone know what position the fuel gauge needle goes to when unplugged? My tachometer just quits sometimes, and my fuel gauge twitches between 1/4 & 1/2 a tank. Just thinking that maybe the rear ending loosened the harness in the back of the cluster.

Thanks again everyone,
Jeff M.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Smells like a bad ground. Try that first.

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1986 300E 5-Speed 240k mi.

The reason for flushing the system with fresh fluid is that the brake fluid is hygroscopic, meaning it soaks up water. The water in the brake fluid corrodes everything and makes a mess. The mess has particles that then ruin the rubber.

If it was bad enough to leak, it needs flushing NOW. It takes less than an hour and a few dollars worth of fluid to do it properly. If something was awry causing air in the system, you can expect further problems, if you flush now, you may prolong it and prevent even further problems. Just do it.

Good luck,

Had the same thing on another make of car. It was the master brake cylinder. BE CAREFUL. Mine worked again for about 2 weeks after bleeding then acted up again. This happened about 4 times before the total final failure. Your master brake cylinder could be failing. When you drive leave enough room in front to react if it fails in traffic. Be prepared to hit the emergency brake if needed. Mine finally quit forever when I had to hammer the brakes on - they held for a second then the pedal went to the floor - never to work again until the new master cylinder was installed. Forget trying to diagnose while they work - it won't show anything. Good luck

Thanks csnow, I'll give that a shot.

Yea I plan on bleeding out the rear two calipers today, being I already have those wheels off. I also have a bad feeling about how long this hard pedal will last. At least before I still had rear brakes. Does anyone know where a rebuild kit can be found?

Thanks,
Jeff M.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

I'd suggest you get it professionally checked thoroughly atleast once. Go ahead and do the repairs on your own if you like. I wouldnt take any chances when it comes to your safety.
Good luck !

While you can rebuild a MC, it's fussy and time-consuming work.
A rebuild could perhaps be considered fun and rewarding in its own way, but when you consider the labor involved, buying a rebuilt unit is almost always more economical.

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1986 300E 5-Speed 240k mi.